1. Field of the Invention
The invention relates generally to imaging systems or cameras (such as video cameras), and exposure control systems for such systems/cameras.
More particularly, the invention relates to image intensifier cameras (cameras that includes an Image Intensifier Tube, referred to hereinafter as an "I.sup.2 Tube"), and automatic exposure control systems for such cameras.
According to one aspect of the invention, the dynamic range of an image intensifier camera may be extended by using an automatic exposure control system which, in response to imager output (for example, the output of a charge coupled device (CCD) imager included in a solid state image intensifier camera), (a) generates a first control signal that controls the gain of the I.sup.2 Tube itself; (b) generates a second control signal for selectively gating the I.sup.2 Tube on or off; and (c) generates a third control signal for controlling a light attenuator that, according to a preferred embodiment of the invention, is included in the camera. The automatic exposure control system contemplated by the invention has a control range in excess of 10.sup.10 .times..
According to a further aspect of the invention, the automatic exposure control system takes a measurement of the exposure from an imager (again, for example, a CCD sensor) and digitizes it for comparison to a desired setpoint. The measurement used may be a peak, an average or some weighted function. The automatic exposure control logic then adjusts, in order, the I.sup.2 Tube gain, the I.sup.2 Tube gating and the light attenuator (using the aforementioned first, second and third control signals), to adjust for steadily increasing light from the object under observation.
In addition to automatic exposure control systems per se, the invention is also directed to camera systems which incorporate such automatic exposure control systems and to methods for realizing the automatic exposure control systems of the type described hereinafter.
2. Brief Description of the Prior Art
A variety of automatic light control systems for imaging/camera systems are well known in the prior art. In addition, a wide variety such systems that utilize image intensifiers (such as an I.sup.2 Tube) and/or solid state imagers (such as a CCD imager) are also well known.
These systems have many applications, including television camera systems, navigational and tracking systems, etc. For many of these systems it is desirable to have a wide dynamic range of performance which permits system operation over a wide range of changing light conditions, ranging from very low light conditions (e.g., systems which operate at night), to high intensity light conditions.
Several patents are briefly described hereinafter for background purposes and to illustrate the state of the prior art.
Prince et al., in U.S. Pat. No. 4,050,085, describes a mechanical system for automatically regulating light input to an electro-optical imaging system used in tracking and navigational systems. According to Prince et al., light measurements, that include peak, average and weighted measurements, are taken and used for controlling an I.sup.2 Tube, a vidicon with adjustable gain, a mechanical iris controller and a number of amplifiers.
Gilligan et al., in U.S. Pat. No. 4,202,014, describes a pulse modulated automatic light control system for a television camera.
The use of CCD or solid state imaging devices in place of film in cameras is described in Ochi et al., U.S. Pat. No. 4,541,016. A further example of a prior art CCD or solid state imaging device is disclosed in U.S. Pat. No. 4,851,914, to Pfanhouser et al., which describes a high speed full frame imaging CCD camera.
Woolfolk, in U.S. Pat. No. 4,872,057, describes an exposure control system for a TV camera which uses a gated I.sup.2 Tube to enable low light television systems to be used over a wide dynamic range of light input.
Lam et al., in U.S. Pat. No. 4,918,534, describes an image processing system which includes an I.sup.2 Tube, an LCD attenuator, a TV camera and a brightness control circuit which controls the attenuator.
Gilligan et al., in U.S. Pat. No. 4,985,773, discloses an exposure control system for a TV camera which includes an I.sup.2 Tube controlled using an amplitude adjustable, periodically pulsed signal.
Wijen, in U.S. Pat. No. 5,101,275, describes a video camera with light intensity control means including an adjustable light attenuating liquid crystal plate.
Yoshimura et al., in U.S. Pat. No. 5,140,424, describes a solid state video camera (CCD based) that includes noise reduction signal processing apparatus.
Seppi et. al., in U.S. Pat. No. 5,168,532 describes a method for improving the dynamic range of an X-ray camera which includes an IIT. The video signal is digitized for further processing in the control circuitry.
Alford et al., in U.S. Pat. No. 5,233,428, hereby incorporated by reference, discusses (in the "Related Art" section of this reference) the use of CCDs in camera images to enable the production of smaller and more durable camera imagers; the effort spent on developing exposure control systems for effectively controlling imager sensitivity to incident light on the imager (that is, controlling the amount of charge a CCD accumulates during a field integration period); prior art exposure control systems which relied on (i.e., are implemented using) mechanical devices (such as a mechanical iris/or mechanical shutter wheel, etc.); relatively large vacuum tube type imagers; on chip shuttering options which allow accumulated charge in the charge storage wells of a CCD to be dumped before the end of a normal integration period, etc.
In addition to the outlining of systems exemplifying the state of the art, the incorporated reference discloses a high performance exposure control system for effectively controlling CCD light sensitivity under varying light conditions. In particular, the Alford et al. reference teaches controlling CCD light sensitivity by electrically controlling a CCD such that the CCD discharges the charge resultant from incident light for a percentage of a field integration period. In other words, the incorporated reference electronically varies the CCD's exposure time.
The technique proposed by Alford, et al. for controlling exposure in their solid state imager, namely electronically controlling CCD exposure time directly, has a potentially negative impact on imager tube life (for example, I.sup.2 Tube life), which is a major contributor to imager failure rates. The failure mechanism of these devices is associated with the average current, which is not held constant by Alford at al., and similar systems.
Furthermore, the use of tube current for exposure level detection does not lend itself towards tailoring the exposure control loop to various applications in which it may be desirable to emphasize, de-emphasize or even ignore portions of a scene contributing to a desired exposure level.
Still further, none of the exemplary prior art discussed hereinbefore provides a wide range exposure control mechanism for image intensifier cameras, where wide range is defined as a control range in excess of 10.sup.10 .times..
Accordingly, it would be desirable to provide image intensifier cameras having a wide dynamic range of performance which permits system operation over a wide range of changing light conditions, ranging from very low light conditions (e.g., systems which operate at night), to high intensity light conditions.
In particular, it would be desirable to provide methods and apparatus for being able to extend the dynamic range of image intensifier cameras to a control range in excess of 10.sup.10 .times..
Furthermore, it would be desirable to provide methods and apparatus for controlling the dynamic range of image intensifier cameras, in particular those cameras utilizing I.sup.2 Tubes, which prolong tube life by directly controlling the image intensifier tube itself (as opposed to controlling, for example, CCD exposure time as performed by Alford et al.).
Still further, it would be desirable to provide methods and apparatus for controlling the dynamic range of image intensifier cameras which allows the response of the exposure control loop to be tailored to various applications in order, for example, to allow specific parts of the scene can be emphasized, de-emphasized or ignored totally as they contribute to the desired exposure level.